1. Field of the Invention
The present invention generally relates to cooling systems. The present invention more specifically relates to cooling systems for high power battery packs.
2. Description of the Related Art
A vapor compression system as known in the art generally includes a compressor, a condenser, and an evaporator. These systems also include an expansion device. The combined weight of these elements greatly reduces their effectiveness for cooling batteries, due to the portable nature of battery powered devices.
In a prior art vapor compression system, a gas is compressed whereby the temperature of that gas is increased beyond that of the ambient temperature. The compressed gas is then run through a condenser and turned into a liquid. The condensed and liquefied gas is then taken through an expansion device, which drops the pressure and the corresponding temperature. The resulting refrigerant is then boiled in an evaporator. This vapor compression cycle is generally known to those of skill in the art.
FIG. 1 illustrates a vapor compression system 100 as might be found in the prior art. In the prior art vapor compression system 100 of FIG. 1, compressor 110 compresses the gas to (approximately) 238 pounds per square inch (PSI) and a temperature of 190° F. Condenser 120 then liquefies the heated and compressed gas to (approximately) 220 PSI and 117° F. The gas that was liquefied by the condenser 120 is then passed through the expansion valve 130 of FIG. 1. By passing the liquefied gas through expansion valve 130, the pressure is dropped to (approximately) 20 PSI. A corresponding drop in temperature accompanies the drop in pressure, which is reflected as a temperature drop to (approximately) 34° F. in FIG. 1. The refrigerant that results from dropping the pressure and temperature at the expansion valve 130 is boiled at evaporator 140. Through boiling of the refrigerant by evaporator 140, a low temperature vapor results, which is illustrated in FIG. 1 as having (approximately) a temperature of 39° F. and a corresponding pressure of 20 PSI.
The cycle related to the system 100 of FIG. 1 is sometimes referred to as the vapor compression cycle. Such a cycle generally results in a coefficient of performance (COP) between 2.4 and 3.5. The coefficient of performance, as reflected in FIG. 1, is the evaporator cooling power or capacity divided by compressor power. It should be noted that the temperature and pressure references that are reflected in FIG. 1 are exemplary and illustrative.
FIG. 2 illustrates the performance of a vapor compression system like that illustrated in FIG. 1. The COP illustrated in FIG. 2 corresponds to a typical home or automotive vapor compression system—like that of FIG. 1—operating in an ambient temperature of (approximately) 90° F. The COP shown in FIG. 2 further corresponds to a vapor compression system utilizing a fixed orifice tube system.
Such a system 100, however, operates at an efficiency rate (i.e., COP) that is far below that of system potential. To compress gas in a conventional vapor compression system 100 like that illustrated in FIG. 1 typically takes 1.75-2.5 kilowatts for every 5 kilowatts of cooling power generated. This exchange rate is less than optimal and directly correlates to the rise in pressure times the volumetric flow rate. Degraded performance is similarly and ultimately related to performance (or lack thereof) by the compressor 110.
Haloalkane refrigerants such as tetrafluoroethane (CH2FCF3) are inert gases that are commonly used as high-temperature refrigerants in refrigerators and automobile air conditioners. Haloalkane refrigerants have also been used to cool over-clocked computers. These inert, refrigerant gases are more commonly referred to as R-134 gases. The volume of an R-134 gas can be 600-1000 times greater than the corresponding liquid.
In light of the theoretical efficiencies of systems using haloalkanes or other fluids, there is a need in the art for an improved cooling system that more fully recognizes system potential and overcomes technical barriers related to compressor performance. There is also a need for cooing systems with reduced weight.